Rejection of a Colletotrichum trifolii Isolate from Red Clover for Biological Control of Black Medick on the Basis of Pathogenicity to Cultivated Clovers

In a search for potential biological control agents for weeds, an isolate of Colletotrichum trifolii originating from red clover seeds from Belleville, Ontario, was tested for host specificity and evaluated for control of black medick (Medicago lupulina). Under controlled conditions, this isolate was most pathogenic on red clover (killing 50 to 90% of the plant tissue) and less pathogenic on alfalfa, black medick (killing 20 to 40%) and yellow sweet clover (killing 10 to 15% of the plant tissue). Inoculated plants of these four species all showed significantly reduced biomass. Other leguminous species, white clover, alsike clover, berseem clover, strawberry clover, bean, faba bean, pea, lentil, lupine, lathyrus, goldenbean, birdsfoot trefoil, and northern joint-vetch, showed slight foliar symptoms, but there were no observed effects on biomass. In temperature studies ranging from 0 to 45°C, this isolate had its optimum mycelial growth at slightly above 25°C, but grew well between 15 and 35°C, indicating that it is adapted to southern Canadian conditions. Based on the less-than-adequate effect of this C. trifolii isolate on black medick (<70% control), in combination with the risk of damage it could pose to alfalfa and other clovers if it were applied on a large scale in areas where those forage crops are grown, this isolate was not pursued further for biological control of black medick.